The purpose of the bone mineral density in childhood study (BMDCS) is to obtain standard reference bone mineral density measures taking into consideration skeletal maturation, bone size and volume, body size, and pubertal stage. These data will serve as an extremely valuable standard pediatric BMD database. It is anticipated to become the essential reference that will enable pediatricians to study, diagnose, and treat children with disorders affecting bone accrual. The six contracts for the Clinical Centers and the Data Coordinating Center were awarded on August 1, 2001. The five academic clinical centers all adhere to a common longitudinal observational protocol involving a baseline and three consecutive annual evaluations, including dual x-ray absorptiometry (DXA) and bone age x-ray, a physical exam to determine their pubertal status, stadiometer height, weight, and body mass index (BMI). Additionally, three sites are performing CT or pQCT scans on a subset of children. Information relating to bone health such as diet and exercise is obtained from each child annually. The bone age x-ray and DXA scans are interpreted centrally. Recruitment commenced in July 2002 at five Clinical Centers and resulted in the enrollment by November 1, 2003 of 1554 children from 6-16 years old of both genders and all racial groups. Specific Aims: The purpose of this work is to complete the reference data and to provide information to establish whether DXA values obtained in early puberty are predictive of bone mineral density values at sexual maturity. For example, a child who enrolled into the BMDCS at age 9 and followed for seven years to age 16 may provide data from pre-puberty through full pubertal maturity. These reference data will answer this very important question and will also allow us to determine our ability to track bone mass using DXA measures and to aid in the identification of young children who may be at risk for developing osteoporosis later in life. The ultimate goal is to develop pediatric reference curves for BMD. CT and pQCT scans will be generated by two clinical centers during the current contract period will provide additional longitudinal data for this study at three study visits. This provides the needed information three dimensional view of the BMD not provided by the DXA alone. CHLA, CHOP and Cincinnati are the three sites involved with this substud